This invention relates to an electrical motor control for an electric motor and more particularly a start-up switch and a protector which is associated with the start-up switch to be installed on the compressor unit for use in a refrigerator, etc.
In the case of a compressor to be used in the refrigerator or the like, it is usually the case that there is provided a start-up switch for supplying start-up torque to the rotor in a single phase motor during the start-up of the motor. This start-up switch is typically constructed in such a way to include a resistive element that has a characteristic of increasing its resistance value with the elevation of its temperature. This switch is connected in series with the auxiliary winding that is connected in parallel with the main winding of the motor.
Such a start-up switch according to prior art is designed to give start-up torque to the rotor of the motor due to the flow of a prescribed electric current to the auxiliary winding at the time the temperature of the resistive element is low and accordingly its resistance value is also low. After the start-up of the motor, however, the resistive element starts to generate heat due to the electric current that flows through the auxiliary winding, which results in an increase in its resistance value. As a result, the electric current that flows to the auxiliary winding gradually decreases, with a result that a constant electric current starts flowing to the main winding.
In the case of such a prior art start-up switch, however, the electric current continues to flow, albeit at a low level, to the auxiliary winding of the motor, with the result that electrical power is unnecessarily wasted in the auxiliary winding subsequent to the start-up of the motor.
It is an object of the present invention to provide a simple, reliable, compact and economical device, which is capable of reducing the electric power consumed by a motor.
Briefly, an electrical motor control for an electric motor made in accordance with this invention, comprises a start-up switch electrically connected in series with the auxiliary winding which includes a resistive element having a characteristic that its resistive value increases with an increase in temperature and a thermostatic switch having a bimetallic actuation member which is responsive to temperature so that upon reaching a preselected thermostatic switch actuation temperature said member moves from a closed current path position to an open current path position, which removes electric power from said auxiliary winding, said thermostatic switch being positioned to be in direct heat transfer relationship with said resistive element so that said actuation member will raise in temperature as said resistive element raises in temperature to a temperature above said preselected thermostatic switch actuation temperature.
In another aspect of the invention, the thermostatic switch remains in the open current path position even after the resistive element of the start-up switch has cooled down. This object is obtained by positioning the thermostatic switch in direct heat transfer relationship with the motor and providing the preselected thermostatic switch actuation temperature to be less than the operational temperature of the motor after start-up.
In yet another aspect of the invention, both the start-up switch and motor protector are contained in the same package which is mounted on the three terminal pins of the motor for a compact, efficient design.